1. Field of the Invention
The present invention concerns a device for forming a film on a substrate in a vacuum and, more in particularly it relates to a vacuum arc vapor deposition device utilizing vacuum arc discharge for the evaporation of film materials.
2. Description of the Prior Art
The vacuum arc vapor deposition is a process of vaporizing a film material from an evaporation source (cathode) by vacuum cathodic arc discharge in a vacuum chamber and depositing it on a substrate. Atoms of the cathode material are emitted as a plasma beam at a high energy by a high arc current from the cathode as an evaporation source and accelerated, if necessary, by a negative voltage applied to the substrate, to form a film on the substrate.
One of the features of the vacuum arc vapor deposition process is a high ionizing ratio and high energy of the atoms of vaporized cathode material. Owing to this, a film with high density and excellent strength and durability can be obtained. Further, it has an industrially noted feature of high filmforming rate and excellent productivity.
Another feature of the vacuum arc vapor deposition process is that the film forming property is stable. That is, in the vacuum arc vapor deposition process, a constant amount of evaporation can be obtained in proportion to the current of vacuum arc discharge and, under predetermined processing conditions, the reproducibility upon repeating processing is extremely high. Therefore, in the vacuum arc vapor deposition process, control, of the thickness of the film for example can be attained sufficiently by controlling a product of the arc discharging current supplied to an evaporation source and a discharge duration time. Furthermore, it has also been known to control the thickness of a film by a value obtained by accumulating with time an arc current supplied to the evaporation source (on the basis of unit, such as coulomb or A Hr).
However, even in the vacuum arc vapor deposition process with stable film property, change of property such as reduction of the film-forming rate appears, although gradually, in the course of consumption of an evaporation material that constitutes the cathode (referred to as a target) to its life which is generally as much as several tens of lots. For instance, according to the results of an experiment conducted by the present inventor, when a target made of titanium initially having an outer shape of 100 mm .times. 16 mm thickness is consumed by about 50% (by weight) and approaches its life, the film-forming rate was reduced by about 15% as compared with the initial stage. Accordingly, in the course of processing several tens of lots, the film thickness during processing is reduced by 15% under constant film-forming conditions. Further, the change of the evaporation property gives an effect not only to the film thickness but also to the nature of the film formed.
In the existing batchwise processing devices, if the change of the film thickness is within an allowable range, correction has been scarcely applied as a countermeasure for the fluctuation of the evaporation property. If the correction is necessary, an operator partially changes processing conditions considering the degree of the consumption of the cathode or by judgment in view of experience based on the number of lots processed. Further, as another method, the film thickness of processing products has been periodically monitored and fedback to processing conditions.
The above-mentioned situations are the same also in a device capable of automatic operation and, in a case where the change of the evaporation property due to the consumption of the cathode gives rise to a problem, a plurality of processing conditions are previously provided and they are applied switchingly by an operator in accordance with the consumption of the cathode.
Naturally, change of the evaporation property which changes the film property is not desirable but, in a case where an operator switchingly applies the processing conditions for correcting such a change, it brings about such a problem that a manual error or individual difference among operators is liable to occur.
Further, in a continuous type device for mass-processing such as a highly automated vacuum arc vapor deposition device, the problem becomes more severe. That is, such a device has an independent vacuum exhaustion chamber, a vapor deposition chamber and an atmosphere releasing chamber, and can process several tens of lots per day, during which processing is applied without operators' aid. In such a case, since necessary correction for the processing conditions can not be conducted, scattering of film thickness may be caused between each of the processing lots.